The present invention relates to a method for fabricating solar cells, and more particularly, a method for fabricating solar cells on flexible substrates.
The conversion of light into electricity occurs over a wide variety of photovoltaic materials. For example, photovoltaic materials include single element semiconductors, such as amorphous silicon and thin-film crystalline silicon, as well as compound semiconductors, such as cadmium telluride (CdTe) and copper indium gallium (di)selenide (Cu(In,Ga)Se2 or CIGS). These and other photovoltaic materials absorb photons of energy sufficient to create electron-hole pairs across a junction, thereby creating an internal electric field. The internal electric field results in a buildup of voltage between two electrodes to provide a source of electrical power.
In the fabrication of solar cells, it is often advantageous to gradually anneal the photovoltaic material to thereby improve its photovoltaic efficiency, defined as electrical power output divided by irradiance. A typical annealing process involves the insertion of a photovoltaic precursor into a furnace, together with a substrate, for a period of time. For example, a known annealing process for a CIGS solar cell includes the exposure of a CIGS photovoltaic precursor supported by a glass substrate to high heat for up to several hours. According to this known process, a CIGS precursor material is deposited by close space sublimation or liquid vapor transport and is annealed at temperatures in excess of 600° C. The resulting solar cell can have a photovoltaic efficiency greater than 16% owing in part to the improvement in grain boundaries and grain size during the annealing process.
Annealing the photovoltaic material can improve its photovoltaic efficiency in a number of ways. For example, an annealing step can cause dopants to diffuse or migrate in the material in a controlled manner. In addition, an annealing step can produce grain growth or coalescence of the photovoltaic material and can heal defects in the photovoltaic lattice. In many instances, however, it is desirable to include a flexible substrate, such as a low-cost polymer, in place of the rigid glass or quartz substrates common in the art. Many polymer substrates have an upper operating temperature (i.e., the temperature at which the material will degrade or decompose) well below 400° C., while most photovoltaic precursors are processed according to conventional methods at temperatures in excess of 600° C. As a result, low cost manufacturing approaches such as roll-to-roll techniques are often not permitted due to the high temperature processing required of most photovoltaic precursors. Even where the solar cell includes a thermally-insulating layer between the substrate and the photovoltaic precursor, conventional heating methods can damage both the substrate and the substrate-precursor interface. In the absence of an annealing step, however, the resulting efficiencies can be poor. For example, an untreated CdTe precursor applied to a polymeric substrate by sputter deposition or vapor deposition at room temperature typically results in solar cells with efficiencies of less than 1%.
Therefore, there remains a need for a high-throughput, low-cost method for manufacturing thin film photovoltaic materials on flexible, large area substrates. In particular, there remains a need for an improved process for annealing a photovoltaic precursor deposited on a flexible substrate to leverage the benefits of thin film photovoltaic materials across a wide range of applications.